In the manufacture of patterned structures, such as wafer level packaging, electrochemical deposition of electrical interconnects has been used as the density of the interconnects increase. For example, see Solomon, Electrochemically Deposited Solder Bumps for Wafer-Level Packaging, Packaging/Assembly, Solid State Technology, pages 84–88, April 2001; disclosure of which is incorporated herein by reference. Wafer level packaging produces a chip/die/device that is ready for direct assembly onto the final substrate or final system platform. Wafer-level packaging is used for making electrical connections to an integrated circuit chip above the active circuitry and is especially important as the density of inputs and outputs (I/Os) on chips increases.
Wafer-level packaging schemes use a technique known as redistribution to connect the peripheral pads to an area array of solder bumps on the surface of the wafer. The basic sequence of wafer-level packaging with redistribution involves creating a level of interconnect that defines an under-bump pad that is connected to the peripheral bonding pad. The under-bump pad is exposed by a via in a dielectric layer. Then the entire wafer receives an under-bump metallurgy (UBM) stack that provides an electroplating seed layer on top of a diffusion barrier and adhesion layer. The plating mask is formed in photoresist that can range from about 1 μm to over 200 μm thick, but are typically 25–125 μm thick. Layers exceeding about 100 μm to about 125 μm are typically applied in double coats. The solder bump is electroplated within the via in the case when a thicker photoresist is used. The solder bump is typically electroplated above the photoresist when it is <50 μm thick (overplating or mushroom plating). The photoresist is then stripped and the UBM is etched away everywhere it is not covered by the solder bumps. Finally, the bumps are reflowed, causing them to reform in the shape of truncated spheres.
Gold bumps, copper posts and copper wires for redistribution in wafer level packaging require a resist mold that is later electroplated to form the final metal structures in advanced interconnect technologies. The resist layers are very thick compared to the photoresists used in the IC manufacturing. Both feature size and resist thickness are typically in the range of 5 μm to 100 μm, so that high aspect ratios (resist thickness to line size) have to be patterned in the photoresist.
Photoresists of the present invention also have use in flat panel displays when added pigments are added to the compositions herein.
Concerning the photoresist, photopolymerizable compositions for imaging have long been known in the art. For instance, U.S. Pat. No. 2,760,863 suggested using photopolymerizable compositions in the preparation of printing plates. More recently, similar compositions have been suggested for color proofing systems for the print industry. Since then, a variety of such photopolymerizable compositions have been described and the use of photopolymerizable compositions are now used in microlithography.
The pattern is formed by imagewise exposing the resist material to irradiation by lithographic techniques. The irradiation employed is usually X-ray, UV radiation, electron beam radiation or ion-beam radiation.
The compositions of the present invention are negative-working photoresist compositions. Negative-working photoresist compositions are exposed imagewise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to such a solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating, thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited.
Aqueous developable photopolymerizable compositions are of especial interest for negative working photoresist compositions. The polymeric binders for such compositions can contain acidic functionality so that the binder polymer is soluble in alkaline aqueous solution and thereby renders the photopolymerizable composition developable in alkaline aqueous solutions.
In the literature, carboxylic acid functionalized acrylic polymers are used as the binder polymers. To ensure the physical performances of the imaged patterns, high molecular weight polymers were utilized which may render slower development rates.
The photopolymerizable compound(s) are typically ethylenically unsaturated monomers and/or short chain oligomers which will crosslink by photo-induced free radical polymerization and form desired insoluble patterns.
However, it has been observed that photopolymerizable formulations which exhibit acceptable development rates suffer from the presence of photoresist residue or scum after development into imaged or patterned areas and/or reduced image resolution, especially when the pattern is relatively thick (e.g., greater than 50 μm).